1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Description of Related Arts
Rack-and-pinion steering apparatuses for providing a manual steering force caused by steering a steering member for a rack shaft via a pinion shaft include electric power steering apparatuses for providing an auxiliary steering force generated by an electric motor for a rack shaft.
For example, there have been provided so-called rack-assist electric power steering apparatuses which comprise a speed reduction mechanism including a first bevel gear attached to an output shaft of a steering assisting electric motor and a second bevel gear arranged around a rack shaft and engaged with the first bevel gear, so that a rotation of the second bevel gear is transformed into an axial shift of the rack shaft via a ball screw mechanism.
In recent years, a high level of silence has been demanded in electric power steering apparatuses.
However, when a load is applied to the first and second bevel gears of the speed reduction mechanism, the engagement between both bevel gears (so-called tooth contact) is deteriorated owing to flexure of the shafts that support the gears and teeth themselves. As a result, an abnormal sound owing to engaged sliding occurs, which causes noise.
Therefore, as a so-called pinion-assist electric power steering apparatus, which provides a pinion shaft with an assisting force, there has been provided an electric power steering apparatus for which sound deadening is attained by using a speed reducer including a worm and a worm wheel and by providing a synthetic resin-made tooth portion for the worm wheel (see Japanese Registered Utility Model Application No. 2556890, for example).
In addition, as a gear unit of hypoid gears, bevel gears or the like, there has been provided a gear unit for which, in order to reduce noise by improving the tooth contact, one or two directional components of three axial components of alignment regarding the shaft center of one gear are displaced by a predetermined correction amount (see Japanese Published Unexamined Patent Application No. H10-38036, for example).
In conical gears such as bevel gears, with only an adjustment of alignment as described above, an effect of an improved tooth contact is small. In addition, the tooth contact may become uneven left to right.
It is an object of the present invention to provide an electric power steering apparatus which can secure an appropriate tooth contact between conical gears engaged with each other and can reduce noise.